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Selecting a recurve bow length

It is important to select the correct length of recurve bow to match your draw length. If the bow is too short it will stack around the full draw length and the clicker will be more difficult to use. If the bow is too long, you will be giving away arrow energy, and hence will get greater drift.
A very good way to select the bow length is to measure the bow's force-draw curve.
The attached plot is of Erica's Hoyt (66" with G3 limbs). The curved blue line is the bow's force-draw curve. The short vertical black line shows full draw.
The long black line is drawn from the brace height position to the full draw position. You can see that this line does not cross the force-draw curve and hence the bow does not stack. In fact, Erica could even use a shorter bow without getting stacking. This bow will be nice to use around Erica's draw length, and it will be easy to use the clicker. You can also see that for this bow the stored energy is quite good (the blue line is well above the black line for most of the draw length - hence Erica can expect good arrow energy).
If the black line did cross the force-draw line and we had stacking, Erica would have a more challenging time using the clicker well, and the bow's efficiency would also be reduced. Also, we would then be working the limbs outside their design range, and may be over-stressing them.
This particular bow is hence quite well suited to Erica, and the limbs seem to be working excellently (the designer - gt - has done an excellent job, it is one of the better recurve force-draw curves I have seen, and I have measured many over the past 40 years).
Note that we cannot assume from this that a different bow of that same length will work just as well for Erica - the designer might not have done as good a job as gt and a similar length bow of another make might stack.
It does take a bit of work and care to make these measurements, but in my experience they are well worthwhile.

okay basically when you get your limbs, they're rated. say 70" at 32 pounds. for example.

since the draw length for 70" limbs is 29", so that means if you draw exactly 29", you'll be pulling EXACTLY 32 pounds. For every inch you pull after that, you are pulling an extra 2 pounds (on average, not exact.) that is called stacking.

so which means that if you use 66" limbs when you're having a 29" draw length, theoratically you should be pulling 3(inches) * 2(pounds) = 6 pounds more. though it's not exact. (im' sure ppl with 29" drawlength would have the sense to go for longer limbs)

correct me if i'm wrong. this is what i learnt from my club ppl.. information passed down quite a few generations. hahhaa.

"Stacking" means that the draw force starts to increase quite rapidly (faster than that straight black line). It is sort of like the compounder's "wall" (but that is a rather horrible analogy as the "wall" is much steeper than "stacking"). It is where the bow is past its design draw length.
I measured that force-draw curve using an Easton Bow Mapper, which does that job excellently. You can also do it just with a tape measure and bow scale, but that takes a lot more time and is not as precise.

Here is a recurve bow that stacks. You can easily see that the black line crosses the force-draw line, well before full draw. This bow will be more difficult to shoot than Erica's bow, and not as efficient. Note that the force-draw curve turns up sharply just prior to full draw - using the clicker will be more difficult than for Erica's case. If full draw was about an inch less the bow would not stack. In this case the archer should either use different model limbs or longer limbs.

Leighton,
The black line is simply drawn from the brace position to the full draw position (the draw length and weight that the archer is using). If you change the draw length the black line changes - so if in the stacking case above the archer had a draw length of about 1" less the bow would not stack.

okay basically when you get your limbs, they're rated. say 70" at 32 pounds. for example.

since the draw length for 70" limbs is 29", so that means if you draw exactly 29", you'll be pulling EXACTLY 32 pounds. For every inch you pull after that, you are pulling an extra 2 pounds (on average, not exact.) that is called stacking.

so which means that if you use 66" limbs when you're having a 29" draw length, theoratically you should be pulling 3(inches) * 2(pounds) = 6 pounds more. though it's not exact. (im' sure ppl with 29" drawlength would have the sense to go for longer limbs)

correct me if i'm wrong. this is what i learnt from my club ppl.. information passed down quite a few generations. hahhaa.

That's not actually quite correct...
I thought all limb lengths were measured at 28" (by Hoyt at least; Yamaha used to use another length for example).

Anyway, we'll use mine as an example; it's 44lb at 28" draw, and 68" long. I'm drawing about 29.5", so it's weighing in at about 47lb or so (using that figure of 2lb/inch). This is fine and it doesn't seem to be stacking.

Now if I was drawing 32", the draw weight would go up much faster than 2lb/inch for the last inch or two. This is the upturn at the end of Jim's graph; it's a Bad Thing because it makes the bow much harder to shoot because you're drawing a lot more weight, but you get very little benefit. This is because the energy stored in the bow (and mostly imparted to the arrow) is the area under that force-draw curve (for those that know some calculus, it's an integral ) - if it turns up only at the end there's obviously very little extra energy produced as a result.
Drawing over the quoted length isn't necessarily stacking; they just quote at an arbitrary length because they've got to pick somewhere. For a 70" bow you'd need to be over 30" (hopefully quite a bit over) before stacking was an issue.

Incidentally, this is why longbows feel much heavier than recurves; that force-draw curve of Erika's is convex from beneath, whereas a longbow one is more concave, so you're getting a bit of a stacking effect the whole time.

Incidentally, this is why longbows feel much heavier than recurves; that force-draw curve of Erika's is convex from beneath, whereas a longbow one is more concave, so you're getting a bit of a stacking effect the whole time.

That is true for many longbows, but not all. I have measured the force-draw for my Woomera longbow (made by John Clark) and it is actually pretty good, and of similar shape as a recurve bow (but not so pronounced). hence, it is reasonably fast and not as challenging to manage around full draw as some (but still a challenge because it is pretty strong).

Here is the force-draw curve for one on my longbows.
As you can see, it is similar to a good recurve and is not stacking (just not) at my draw length. It is not as efficient at storing energy as Erica's bow (the curve is not as far above the straight line). This longbow is 48 pounds at 28", but my draw length is longer than that at about 29", so while John has specified this bow at 28" it is still quite useable at 29", but would be stacking after that. The bow is 68" long.
As longbows go, this is a very good force-draw curve.

Leighton,
The black line is simply drawn from the brace position to the full draw position (the draw length and weight that the archer is using). If you change the draw length the black line changes - so if in the stacking case above the archer had a draw length of about 1" less the bow would not stack.

I get it now. The black line represents a perfectly linear weight increase.

Wonder whether there's an easier way to tell whether a bow's the right length like in billiards. About the cue being a certain height with relation to the shoulder.

But then again, with all the different types of limbs producing different draw force curves and with different riser / limb length combinations, that'd be difficult.

James / Marcus or Erika (if you read this), just out of curiosity, what is Erika's drawlength? I'm a little on the long side here. Drawlength of a teeny weeny bit over 27 inches but with a 68 inch bow.